Why MLCCs howl? How to solve?

2023-12-22

MLCC - multilayer chip ceramic capacitor, referred to as chip capacitor, can cause noise whistling problems.

With the popularization of laptops, mobile phones and other equipment, more and more people are interested in researching the "howling" problem caused by capacitor vibration. How to optimize the capacitor whine of each power supply architecture and shut up the capacitor is an interesting question.

Sound originates from the vibration of objects, and sound waves with a vibration frequency of 20Hz~20kHz can be recognized by the human ear.

MLCC emits a whistling sound, which means that MLCC vibrates with a large amplitude (microscopically large, less than 1nm) under the action of voltage.

Why does MLCC vibrate?

Firstly, let’s understand a natural phenomenon - electrostriction.

Under the influence of an external electric field, all materials will undergo stretching deformation - electrostriction.

For some ferroelectric materials with high dielectric constant, the electrostrictive effect is severe, which is called the piezoelectric effect.

Piezoelectric effect includes positive piezoelectric effect and reverse piezoelectric effect.

Positive piezoelectric effect

When mechanical pressure is applied to a dielectric material with piezoelectric properties, the structure of the dielectric crystals will be reorganized and charges will be induced on the surface of the material, resulting in a potential difference.

Inverse piezoelectric effect

When a voltage is applied to a dielectric material with piezoelectric properties, mechanical stress is generated and deformation occurs.

What is piezoelectric effect?

“When pressure, tension and tangential force are applied to a crystal without a center of symmetry, dielectric polarization proportional to the stress will occur, and positive and negative charges will appear on both ends of the crystal. This phenomenon is called the positive piezoelectric effect. . On the contrary, applying an electric field to the crystal causes polarization, which produces deformation or mechanical stress proportional to the electric field intensity. This phenomenon is called the inverse piezoelectric effect. These two positive and inverse piezoelectric effects are collectively called the piezoelectric effect.

Ceramic dielectric is the main component of MLCC, and electrostriction is inevitable under the action of voltage. If electrostriction is strongly manifested as a piezoelectric effect, vibration will occur.

All MLCCs will howl?

The two main materials when design ceramic dielectric are paraelectric and ferroelectric.

Paraelectric media, also known as Class I media, mainly include SrZrO3, MgTiO3, etc. The electrostrictive deformation of paraelectric materials is very small and is not enough to generate noise under the operating voltage. Therefore, MLCCs made of paradielectric (Class I dielectric) materials, such as NPO (COG) and other temperature-stable products, will not produce noise.

Ferroelectric media is also called Class II media, mainly BaTiO3, BaSrTiO3, etc. Ferroelectric media has strong electrostrictive properties - piezoelectric effect. Therefore, MLCCs made of ferroelectric media (Class II media), such as X7R/X5R products, will produce obvious noise whistling under the action of large AC electric field intensity.

As shown above, after a large voltage change is applied to both ends of the X7R-MLCC, the BaTiO3 ceramic produces an inverse piezoelectric effect, and the MLCC deforms and vibrates and is transmitted to the PCB board to resonate.

When the frequency of the voltage signal is within the human hearing range of 20Hz~20kHz, the capacitor can be heard howling.

Which occasions does MLCC whistle obviously?

Larger alternating voltage, frequency between 20Hz and 20kHz, using X7R/X5R medium and high capacity MLCC will produce obvious howling, such as switching power supply, high frequency power supply and other occasions.

Dangers of Howling

Many mobile electronic devices are close to the human ear, such as laptops, tablets, smartphones, etc. If there is audible noise in the electronic circuits, the user experience will be affected. In addition to being annoying, the violent whistling may also be a hidden danger of insufficient reliability design. Violent whistling is caused by violent vibration, and the amplitude of vibration is determined by the degree of piezoelectric effect. The piezoelectric effect is proportional to the electric field strength, and the applied voltage remains unchanged. The thinner the medium, the stronger the piezoelectric effect and the louder the howling sound.

The rated voltage is determined by the material and dielectric thickness of the MLCC. Severe whistling indicates that the dielectric thickness of the MLCC selected is too thin for the current working voltage. An MLCC with thicker dielectric and higher rated voltage should be considered.

For ferroelectric ceramics, under the action of an alternating electric field, there is also the problem of alternating ferroelectric domains to internal friction. The alternating field is strong, the internal friction is serious, and the probability of failure increases. This can be reflected in the size of the howling sound.

How to solve howling?

Many solutions available to reduce the noise produced by MLCC capacitors, but will add cost.

1. Changing the type of capacitor is the most direct method, replacing it with capacitors without piezoelectric effect such as paraelectric ceramic capacitors, tantalum capacitors and film capacitors. However, we must consider the volume space, reliability, cost, etc issues.

2. Adjust the circuit, the most sensitive audio frequency of the human ear is 1KHz~3KHz;

Aim to eliminate the large alternating voltage applied to the MLCC or move its frequency out of the human ear's hearing frequency band.

3. Pay attention to PCB layout and PCB board specifications to help reduce howling levels.

3. Choose noiseless or low-noise MLCC.

Design of Noiseless/Low Noise MLCC

Currently, there are three design solutions for the howling phenomenon of MLCC.

(1) Thicken the bottom protective layer

Since there is no internal electrode in the thickness of the protective layer, the BaTiO3 ceramic in this part will not deform. When the solder height at both ends does not exceed the thickness of the bottom protective layer, the deformation produced at this time will have less impact on the PCB, effectively reducing.

HONGDA Capacitors is quite strong at MLCC, our MLCC stacked up to 600 layers. Capacitance up to 330uF, voltage up to 7200V;

Many case sizes available: 0201 0402 0603 0805 1206 1210 1808 1812 1825 2211 2220 2225.

Lead time: 2weeks, we keep regular items in stock.

(2) Additional metal bracket structure

The structure diagram is as follows. It uses a metal bracket to elevate the MLCC chip.

The MLCC is separated from the PCB board, and the deformation caused by the inverse piezoelectric effect is elastically buffered by the metal bracket, reducing the impact on the PCB board and effectively reducing noise.

(3) Design and manufacture using dielectric materials with weak piezoelectric effect

By further doping barium titanate (BaTiO3) and sacrificing a certain dielectric constant and temperature characteristics, a dielectric material with a greatly weakened piezoelectric effect is obtained. MLCCs manufactured with it can effectively reduce noise.

HONGDA Capacitors can offer you quality MLCC, we own same production equipment with Murata’s, Japan-imported, and support you cost-effective components.